NIH-07 open formula nonpurified diet has been the selected diet for the NTP rodent studies since 1980. This diet was formulated more than 20 years ago for reproduction, lactation and growth of rodents in breeding colonies. This diet which contains approximately 24% protein, 5% fat and 3.5% fiber may not be an optimal diet for rats in long-term studies, and some components such as protein, calcium and phosphorus concentrations may be increasing the severity of chronic diseases. After 6 years of experimentation with different formulations of experimental diets, a new diet designated as NTP-2000 open formula nonpurified diet was formulated for rodents in toxicology studies. This diet is substantially different from NIH-07 diet and contains approximately 15% protein, 8.5% fat and 9.5% fiber. The purpose of this study was to compare 1) growth patterns, 2) hematology and clinical chemistry parameters, 3) organ weights and 4) incidence and severity of lesions in Fischer 344 (F344) rats fed the NIH-07 and NTP-2000 diets for 13 weeks. Groups of 25 male and female F344 rats of 6 weeks of age were fed NIH-07 or NTP-2000 diet for 13 weeks. Feed consumptions and water consumptions were determined and hematology and clinical chemistry evaluations were done at 12 weeks. After 13 weeks of treatment the rats were necropsied, major organs were weighed and histopathologic changes in major organs were evaluated by light microscopy. The growth patterns were essentially same for both diets. The feed consumptions were slightly higher and water consumptions were slightly lower for rats fed NTP-2000 diet. there were no differences in hematological parameters between the groups fed the above diets.The BUN values for males and females fed NTP-2000 diet were significantly lower due to lower protein content of this diet. The cholesterol level in males, and 5'nucleotidase and alkaline phosphatase levels in males and females fed NTP-2000 were slightly higher due to higher fat content of the diet. However, the triglyceride levels in groups fed NTP-2000 diet were lower and it may be related to higher fiber content of the NTP-2000 diet. The liver and kidney weights of male and female rats fed NTP-2000 diet were significantly lower possibly due to lower protein consumption associated changes in drug metabolizing enzymes. The adrenal weights were significantly lower in groups fed the new diet. The NTP-2000 diet prevented nephrocalcinosis, decreased the incidence and severity of nephropathy and cardiomyopathy, the common lesions of F344 rats in 13-week studies. these results indicate that the NTP-2000 diet is adequate for growth and maintenance of rats and appears to prevent or decrease the severity of diet-and-age associated lesions.